1. Field of the Invention
The present invention is directed to the monitoring of motor operation and, more particularly, to a stall detection circuit for two-phase, four-coil stepper motors operating in two-phase-on mode.
2. Description of the Related Art
Two-phase, four-coil stepper motors include a permanent magnet rotor 10 with multiple pole pairs, namely having two stator phases made up of two coils connected in series in each phase, as shown in FIG. 1.
Under typical operation, one H-bridge driver powers each phase. As shown in FIGS. 2A-2C, the standard H-bridge circuit includes a first path having a first switch S1 connected in series with a second switch S2 and, in parallel therewith, a second path having a third switch S3 in connected in series with a fourth switch S4. A pair of coils, coil 1 and coil 2, are connected in series on a branch between a node on the first path between the first and second switches, and a node on the second path between the third and fourth switches. A respective diode D may be connected to nodes on either side of each switch. While the first and second switches are connected in series, in operation of the circuit, the first and fourth switches act in series, and the second and third switches act in series, as explained herein.
Particularly, the standard H-bridge step sequences for forward, reverse and braking modes are depicted in FIGS. 2A-2C, respectively. The circuitry with the third and fourth coils of the second phase is arranged in the same way as that shown for coil 1 and coil 2, only 180 electrical degrees out of phase.
In the forward sequence shown in FIG. 2A, for example, current alternatingly flows through the first switch S1, coil 1 and coil 2, and the fourth switch S4, and then through the third switch S3, coil 2 and coil 1, and the second switch S2. In this configuration, back electromotive force (EMF) generated by the motor is masked by the supply voltage. Transient voltage across the switching coil shows little or no change, regardless of the rotor movement. It is therefore difficult to discern if the rotor has been blocked or rotated in the wrong direction.
Therefore, a need exists for a stall detection circuit providing a method for substantively differentiating normal running mode from a stall condition.